1. Field of the Invention
This invention relates to an evaporated fuel control system for use with an internal combustion engine which is adapted for controlling the amount of evaporated fuel supplied from a fuel tank or an associated fuel supply system to a cylinder or combustion chamber of the internal combustion engine.
2. Description of the Prior Art
A typical fuel supply system of an automotive vehicle comprises a fuel tank equipped with a fuel pump, supply line for supplying fuel from the fuel tank to a fuel injector connected to an end of the fuel supply line which in turn sprays atomized fuel into a combustion chamber. The injector, normally located downstream of the intake air passage close to the cylinder of the internal combustion engine, is directed toward an intake port of the cylinder to spray atomized fuel into the combustion chamber at an appropriate part, e.g., intake stroke, of the engine cycle.
In such a typical fuel system, an upper space of the fuel tank is filled with fuel vapor. This fuel vapor possibly leaks out of the tank through a fuel inflow port. To prevent this leakage, an automotive vehicle is usually provided with an evaporated fuel trap and recycle system. A principal structure of this kind of evaporated fuel trap and recycle system comprises an evaporated fuel line connecting upper space of the fuel tank and an intake air passage, and a canister provided at an intermediate portion of the evaporated fuel line. There further provided a purge valve downstream of the canister. For example, Japanese Unexamined Patent Publication No. 4-136469 discloses such evaporated fuel trap and recycle system.
The canister is generally known as an evaporated fuel absorber or trap device which is usually filled with absorbent such as activated carbon. Evaporated fuel, conveyed from the fuel tank to the canister through the evaporated fuel line, is therefore temporarily trapped by the absorbent in the canister. Thereafter, the fuel desorbs from the absorbent, being propelled by operation of the internal combustion engine. After passing through the intake air passage, the evaporated fuel is introduced into a cylinder of the engine together with intake air. The purge valve is controlled to adjust its opening degree.
Now, recently advanced automotive vehicles have a torque down control system by which the engine is driven with extremely small engine torque during idling operation or when the vehicle is cruising on a descending road. Also, the torque down control is executed in cooperation with an automatic traction control device of the automotive vehicle to suppress slip of wheels promptly. The former torque down control will be executed for fuel saving. The latter torque down control will executed for slip control.
Further, in an internal combustion engine connected with an automatic transmission, to reduce the shifting shock, a torque down control is executed when the gear ratio is being shifted.
As one of the torque down control, fuel cut operation is performed. According to a typical fuel cut operation, an engine is partially fired. Specifically, a specified number of cylinders are excluded from fuel supply. Japanese Unexamined Patent Publication No. 3-67042 discloses a vehicle traction control wherein such fuel cut operation is performed in cooperation with a slip control device.
As another torque down control, the ignition timing is retarded. Specifically, the ignition timing for each cylinder is retarded by a predetermined amount or a predetermined crank angle to decrease the engine torque.
Returning to the evaporated fuel control, a conventional control of evaporated fuel is carried out in such a manner that the opening degree of the purge valve is automatically adjusted in accordance with the engine speed. Alternatively, the purge valve is controlled in accordance with nature of volatile components of fuel as shown in Japanese Unexamined Patent Publication No. 4-136469.
In the torque down control, as mentioned above, the internal combustion engine is not always operated with full cylinders. Some cylinders are forcibly held off or the ignition timing is retarded to accomplish the torque down effect in an idling, slip control, gear ratio shifting or the like. The combustion is unstable in such an engine operating condition.
However, the conventional evaporated fuel supply control is carried out in non-connection with the torque down control. Accordingly, the event has often occurred that the torque down is insufficiently attained, or the after-burning phenomenon is induced undesirably.
It has been well known that the fuel amount sprayed from the injection nozzle can be controlled precisely by regulating the injection nozzle. However, it has been very difficult to finely control the amount of evaporated fuel flowing into the intake passage because of the vapor state. Accordingly, it will be apparent that in such special engine operation condition as torque down control which requires fine fuel amount control, it is preferable to regulate the injection nozzle only while inhibiting the evaporated fuel supply.
In the conventional torque down control of fuel cut operation, some of cylinders are excluded from fuel supply to reduce the engine torque when wheel slip is detected. The fuel supplied to the remaining cylinders is caused to be lean to reduce the total amount of unburnt carbon monoxide or hydrocarbon components in the exhaust gas, and to keep the exhaust gas passage from the after-burning phenomenon even if the exhaust gas is mixed with oxygen rich exhaust gas scavenged from the fuel-cut cylinders, and to suppress the rise of temperature of the exhaust passage. This is advantageous particularly in an engine equipped with an exhaust gas purification system including a catalyst in that the catalyst is liable to deteriorate in high temperature environment.
However, the evaporated fuel is supplied irrespective of the fuel cut operation. Consequently, the evaporated fuel is mixed with the lean fuel-air mixture, and the above-mentioned effect of lean fuel-air mixture is weakened. This is because the air-fuel ratio of the lean fuel-air mixture shifts to a richer value due to the undesirable addition of evaporated fuel. With the undesirable shift of air-fuel ratio, the torque down is no longer accomplished properly or the after-burning phenomenon is likely to occur in the exhaust gas passage. The after-burning phenomenon will cause deterioration of valuable catalyst.
In the conventional torque down control of ignition timing retard, also, the evaporated fuel is supplied into the combustion chamber irrespective of the ignition timing retarding. This will cause the fuel to be rich more than the necessary value, and involve the after-burning phenomenon.